During the Hail Stress Sequence, sample modules are struck by a 50mm lab-manufactured ice ball at terminal velocity (32 meters per second) in 11 different locations at a zero-degree angle. PVEL adds that the simulated hail strikes deliver an impact energy of 31.4 joules, comparable to the approximate impact energy of a 77mm strike with the same density at terminal velocity at 30 degrees.

Generally speaking, laboratory-made hail is extremely dense and more uniform than natural hail, so its impact energy is usually far greater; the impact energy of a lab-made 50mm ice ball is comparable to a natural hailstone as large as 100mm in diameter, depending on their density and impact angle.

PVEL has identified three consistent patterns of hail damage:

First, in strings with two or more broken glass modules, the cells of the remaining modules with intact glass are likely to experience severe cracking.

Second, in sites with a capacity of more than 100MW that suffer glass breakage from hail, it is likely that some areas of the array will not have any cell damage.

Third, glass breakage and cell cracking will be variable in the areas of the site between the two above extremes.

Glass breakage rate of glass-glass modules could reach 89% if they are impacted by hail of 50mm diameter

Breakage rate drops to only 34% for glass-backsheet modules that use fully tempered glass

“As true heat-tempered glass is generally twice as strong as glass that is ‘heat-strengthened’ only, our test data shows that PV modules made with 3.2mm fully tempered front glass are approximately twice as resilient to impact as modules packaged with 2.0 mm heat-strengthened front glass.”

To determine if it was financially viable to stow, stow, stow your solar modules during a hail event:

Under a US$22 per MWh power purchase agreement (PPA), moving into the hail-stow mode during WWA events throughout the year for this particular simulated site resulted in a total production loss of US$12,000, or 0.1% of the US$9.75 million estimated annual revenue.


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